Cotton fiber is the single most important textile worldwide. About 80 million acres of cotton are harvested annually across the globe. Cotton is the fifth largest crop in the U.S. in terms of acreage production, with an average of 10.3 million acres planted in the years 2006 to 2008. About 90% of cotton grown worldwide is Gossypium hirsutum L., whereas Gossypium barbadense accounts for about 8%. Consequently, the modification of cotton fiber characteristics to better suit the requirements of the industry and the consumer is a major effort in breeding by either classical methods or by genetically altering the genome of cotton plants. Goals to be achieved include increased lint fiber length, strength, dyeability, fiber maturity ratio, fiber uniformity, decreased fuzz fiber production, immature fiber content, and micronaire.
Cotton fiber development is a multistage process under the regulation of a vast number of genes, many of which are up-regulated or highly expressed in developing fiber cells (Li, C. H. et al. 2002, Plant Sci 163: 1113-1120; Ruan et al. 2003, Plant Cell 15: 952-964; Wang et al. 2004, Plant Cell 16: 2323-2334; Li et al. 2005, Plant Cell 17: 859-875; Luo et al. 2007, Plant Journal 51: 419-430).
Each cotton fiber is a differentiated single epidermal cell that initiates from the epidermis of the outer integument of the ovule. Approximately half a million fibers are produced per cotton boll, some forming fuzz and some forming lint. Differentiation of an epidermal cell into a fiber requires a change in cell fate, which is a fundamental biological process involving genetic, physiological and developmental “switches”. However, only ˜25-30% of the epidermal cells differentiate into the commercially important lint fibers. The majority of cells do not differentiate into fibers or develop into short fibers or fuzz. Genetic mutations, polyploidy, pollination/fertilization and hormonal regulation can affect the number of cells developing into fibers or alter fiber cell properties (fuzz vs. lint).
The development of cotton fibers starts on the day of anthesis (flowering) and is divided into four distinct but overlapping phases: fiber cell initiation which starts immediately after anthesis and lasts up to 3 days post anthesis (DPA), elongation (3 till 20 DPA), secondary wall biosynthesis (15-35 DPA) and maturation (45-60 DPA) (Basra & Malik 1984, Int Rev of Cytology 89: 65-113; Graves and Stewart, 1988, J. Exp. Bot. 39 (1): 59-69; Ramsey and Berlin, 1976, American Journal of Botany 63 (6): 868-876; Ruan and Chourey, 1998, Plant Physiology 118: 399-406; Ruan et al. 2000, Aust. J. Plant Physiol. 27:795-800; Stewart, 1975, Am. J. Bot. 62, 723-730). The first three stages occur while the fiber cell is alive and actively growing whereas maturation occurs after opening of the boll containing the white fluffy fibers and describes the drying of the mature fibers.
These developmental phases are regulated by the ordered expression of a multiplicity of genes in the fiber cell, a proportion of which is fiber specific and therefore thought to play a major role during fiber development. The promoters of fiber specific genes may regulate gene function by restricting transcription to the fiber cell (Delaney et al. 2007, Plant Cell Physiol. 48(10): 1426-1437).
Various promoters controlling or regulating the expression of such fiber-preferential or fiber-specific genes have been described and also have been exploited to genetically modify fiber characteristics.
E6 was the first cotton fiber gene identified, and the E6 promoter has been used for engineering cotton fiber quality (John and Keller 1996, PNAS 93: 12678-12773). GhRDL1, a gene highly expressed in cotton fiber cells at the elongation stage, encodes a BURP domain containing protein (Li, C. H. et al. 2002, ibid.), and the GaRDL1 promoter exhibited a trichome-specific activity in transgenic Arabidopsis plants (Wang et al. 2004, ibid.). GhTUB1 transcripts preferentially accumulate at high levels in fiber, accordingly, the pGhTUB1::GUS fusion gene was expressed at a high level in fiber but at much lower levels in other tissues (Li, X. B. et al. 2002, Plant Physiol. 130(2): 666-74). Promoters of three cotton lipid transfer protein genes, LTP3, LTP6, and FSItp4, were able to direct GUS gene expression in leaf and stem glandular secretory trichomes (GSTs) in transgenic tobacco plants (Hsu et al. 1999, Plant Science 143: 63-70; Liu et al. 2000, ibid.; Delaney et al. 2007, Plant and Cell Physiol. 48: 1426-1437).
The cotton R2R3 MYB transcription factor GaMYB2 has been shown to be a functional homologue of Arabidopsis GLABRA1 (GL1), a key regulator of Arabidopsis trichome formation. GaMYB2 is expressed in cotton fiber cells at the early developmental stages (Wang, S. et al., 2004, ibid.). Its promoter drives trichome-specific expression also in Arabidopsis and GST headspecific expression in tobacco (Shangguan et al. 2008, J. Exp Botany 59(13): 3533-3542).
U.S. Pat. No. 7,626,081 discloses a cotton seed-specific promoter found in the alpha globulin gene. The promoter Gh-sp is derived from a seed protein gene and is active only in maturing cotton seeds (Song et al. 2000, Journal Cotton Science 4: 217-223).
U.S. patent application 2003/0106089 discloses a gene expressed in a fiber-specific manner and its promoter which is active particularly in very early fiber development.
U.S. Pat. No. 6,211,430, U.S. patent application 2013/0081154, EP patent application Ser. No. 13/189,991, U.S. Pat. No. 6,096,950 and WO 96/40924 disclose promoters derived from members of a multigene family in cotton which all direct expression during late fiber development.
Despite the fact that many promoters known to drive seed-preferential or fiber-preferential expression in cotton plants are available in the art, these promoters may drive expression of associated genes of interest in cotton tissue other than fiber (initiation) cells, potentially resulting in cytotoxicity and low transformation efficiencies. Therefore, a need remains for fiber-preferential or fiber-selective promoters with the capacity to control transcription in developing fiber cells, preferably in a more selective manner. These and other problems are solved as described hereinafter in the summary, detailed embodiments, examples, drawings and claims.